1. Field of the Invention
The invention relates generally to the field of magnetron sputtering vacuum deposition and, more particularly, to sputtering a multi-layer coating stack having an infrared reflective metal layer without overlaying metal primer layers and also relates to the article made thereby.
2. Description of the Presently Available Technology
Sunlight contains light energy which falls generally into three broad regions: ultraviolet, visible and infrared. For many commercial applications, such as the windows of buildings or automobile windows, it is desirable to reduce the amount of energy, i.e., heat, transferred through the window into and/or out of the building or automobile. This heat reduction can be affected by reducing the transmitted light energy from any of these three regions. However, it is typically not practical to remove too much visible light energy, as this adversely impacts upon the ability of persons to see through the window. It is therefore desirable to block out as much of the remaining energy, such as infrared energy, as possible since this will result in the largest reduction of transmitted energy without adversely impacting upon visible light transmittance.
To reduce solar infrared energy transmittance, it is known to deposit infrared reflective metal layers, such as silver, gold, aluminum or copper, on a glass substrate. However, if only an infrared reflective metal were applied, this would result in a mirror-like finish which would also reflect visible light. Therefore, an antireflective layer is usually provided on one or both sides of the infrared reflective layer to produce a substrate which is highly reflective of infrared energy but which is also highly transmissive of visible energy. These antireflective layers are usually formed of a dielectric material, e.g., metal oxides, such as Zn2SnO4, In2SnO4, TiO2, SnO2, In2O3, ZnO, Si3N4 or Bi2O3 to name a few.
The infrared reflective and antireflective layers are typically formed on the glass substrate in a cathode sputtering coater using a technique known in the sputtering art as magnetron sputtering vacuum deposition. The antireflective layer is usually deposited over the substrate by sputtering a metal or metal alloy cathode in a reactive atmosphere, e.g., an oxygen rich atmosphere, to deposit a metal oxide dielectric coating over the glass substrate surface. A cathode made of an infrared reflective metal, such as silver, is sputtered in a non-reactive, e.g., oxygen-free, inert, atmosphere such as argon to deposit an infrared reflective metal layer over the antireflective layer. The oxygen-free atmosphere is used to deposit a metal layer and to prevent oxidation of the infrared reflective metal cathode. To prevent the breakdown of the silver layer by oxidation or agglomeration during the sputtering of a subsequent antireflective layer, a protective metal primer layer, such as copper, niobium, titanium, tantalum, chromium, tungsten, zinc, indium, nickel-chromium alloys or similar metal, is deposited over the silver layer.
An example of the formation of such metal primer layers is disclosed in U.S. Pat. No. 5,318,685, which disclosure is herein incorporated by reference. These metal primer layers are typically on the order of about 10-30 Angstroms thick and are sacrificial. That is, the thickness of the metal primer layers is determined based upon the system coating parameters so that most of the metal primer layer is reacted, i.e., oxidized, during the sputtering of the subsequent antireflective layer. The protective metal primer layer becomes transparent when completely oxidized so that the oxidized metal primer layer does not adversely impact upon the light transmittance and reflective qualities of the coated substrate. However, this subsequent oxidation of the metal primer layer is not easily controlled and it is not unusual for this oxidation to be less than complete. Further, metal atoms from some metal primers tend to alloy with the metal of the infrared reflective metal layer which degrades the interface between the two layers.
While generally acceptable for producing low emissivity coated substrates, there are drawbacks associated with conventional coating methods. For example, for a coated glass which is to be used without further thermal processing or conditioning, if not all of the metal primer layer is oxidized during application of the subsequent antireflective layer, the residual metal primer layer causes a decrease in visible light transmission. Additionally, the amount and thickness of the residual metal primer layer left after application of the subsequent antireflective layer has an effect on the physical properties of the coating, such as the hardness of the coated substrate. Therefore, it is important to apply only as much of the metal primer layer as will be oxidized during sputtering of the subsequent antireflective layer. However, controlling the thickness of the metal primer layer to such a required degree of accuracy, e.g., 10-30 xc3x85, poses a significant process complexity. Accurate thickness control within e.g., an atomic layer is difficult. Also, controlling the oxidation of the metal primer layer is difficult. In addition to the limitations with incomplete oxidation of the primer layer, with conventional coaters, valuable coating space is wasted because of the need for having discrete oxygen-free infrared reflective metal coating zones separate from the oxygen containing antireflective coating zones.
Additionally, if the coated substrate is to be further heat treated, such as for bending, heat strengthening or tempering, the thickness of the metal primer layer must be increased during processing to leave sufficient unoxidized residual metal primer for protection of the silver layer during such subsequent heat treatment. This means that for commercial purposes, two inventories of the coated substrate must usually be maintained, one having a relatively thin, oxidized primer layer capable of immediate use and one having a relatively thicker primer layer with unoxidized metal primer for use after further heat treatment. However, it is not unusual for the coating properties, such as color, transmission and haze, to be adversely affected by subsequent heat treatment of conventional low emissivity coated substrates with even thicker primer layers.
As can now be appreciated by those skilled in the art, it would be advantageous to provide a coating having one or more infrared reflective metal layers without the need of conventional metal primer layers and a method of making same.
A coated article, e.g., an automotive transparency, e.g. windshield or architectural window, made in accordance with the invention has a substrate with an infrared reflective metal layer, for example, a silver layer, deposited over the substrate and a ceramic layer, e.g., an aluminum doped zinc oxide layer, deposited preferably from a ceramic cathode over the silver layer. Additional antireflective or ceramic layers may be deposited below the infrared reflective metal layer or over the ceramic layer.
The invention provides a method of sputtering a multi-layer coating stack having an infrared reflective metal, e.g., silver, over a substrate by sputtering an infrared reflective metal cathode to deposit an infrared reflective layer over the substrate and then sputtering a ceramic cathode, such as an aluminum doped zinc oxide cathode, to deposit a non-sacrificial, ceramic layer over the silver layer. The silver layer and the ceramic layer may each be sputtered in an inert atmosphere containing a low percentage of oxygen, for example, in the same coating chamber of a coater with the oxygen content controlled in a manner to be described to a level to minimize undesired effects on the silver layer. For example, the oxygen content can be regulated to be about 0-20 vol. % to prevent the resistivity of the silver layer from increasing to a non-preferred level, e.g., increasing by an amount equal to about 75% or more of the resistivity of a silver layer of similar thickness sputtered in an inert, non-reactive atmosphere. Additional ceramic or antireflective layers can be deposited below the silver layer or over the ceramic layer.
A coater is also provided for sputtering a multi-stack coating having an infrared reflective metal, e.g., silver, onto a substrate. The coater includes an infrared reflective metal cathode with at least one ceramic cathode located downstream and preferably spaced from and adjacent to the infrared reflective metal cathode. The infrared reflective metal cathode and the ceramic cathode may be located in the same coating zone.